Closed loop geothermal systems typically consist of a heat pump (heating and/or cooling unit), a ground loop heat exchanger (which can be in many different configurations), and one or more pumps used to circulate the heat exchange fluid (which is usually water mixed with antifreeze). During initial system installation, the ground loop must be filled with liquid and purged of all air and debris. This is typically accomplished with an external pumping system known in the industry as a flush cart. The flush cart often consists of a one to two horsepower pump, valves for directing/controlling flow, a large supply container for supplying the flushing liquid, and hoses/fittings for coupling to the geothermal system. Many geothermal system manufacturers provide a packaged unit that consists of the necessary system circulating pump(s) along with valves and exposed ports for connection to a flush cart. The package units are often known in the industry as flow centers, flow controllers, loop pump modules, or pump packs. The valves are often three way, four position valves in the package center that enable the ground loop system to be isolated from the circulating pump(s) and the heat pump during initial system start up and any subsequent maintenance such as replacing a circulation pump. The valves and the exposed ports also allow a flush cart to be coupled to the geothermal system for filling, flushing and purging. Geothermal systems are often categorized as either being pressurized or unpressurized.
During initial system start up of traditional pressurized systems (after the filling, flushing and purging is complete) the return valve on the flush cart is closed, dead heading the pump against the closed valve. This drives up the pressure in the system as the pump draws a small amount of liquid from the flush cart reservoir and forces it into the system. The ground loop often consists of plastic pipe that flexes slightly allowing this additional liquid to enter the system. The flush cart operator watches the flush cart fluid supply container for a significant liquid level drop. If the water level in the supply tank drops significantly (maybe several inches or more) it may indicate that air is still in the system and is being compressed, and further flushing/purging may be necessary. Once the operator is confident that all air has been purged from the geothermal system, the flush cart is once again dead headed and the valves are turned to the operating position capturing the pressure in the system. This initial system pressure, which is provided by the flush cart pump, may be generally on the order of 40-60 psig. This trapped pressure provides the suction pressure required by the circulating pumps, and also allows for some pressure loss due to the relaxing of the plastic piping over time. If all air has been properly flushed from the system, and there are no leaks anywhere in the system, these pressurized geothermal systems will function for many years without any maintenance required.
This method of installation has been the dominate strategy since the geothermal industry was originally founded. However, if any air remains in the system and/or there is a small leak anywhere in the system, the system pressure will drop and the volume of trapped air will grow, due at least in part to the decrease in pressure. This build up of air can cause two modes of failure for the circulating pump(s). First, the air can migrate to the pump inlet and cause the pumps to become air locked. Since the loop fluid cools the circulating pump, an air locked pump will eventually overheat and fail. Second, if there is inadequate positive pressure on the suction side of the pump, cavitation can result and the pump will eventually fail.
Unpressurized systems have been gaining wider industry acceptance over the past several years. Unpressurized systems generally allow the installation to be less precise, thereby allowing a larger group of lesser skilled installers to offer closed loop geothermal heating and cooling systems to their customers. Since there is little to no system pressure relative to ambient pressure, piping connections do not necessarily have to be as leak proof as connections in their counterpart pressurized geothermal systems. Unpressurized systems typically allow for some air separation and often provide a strategy for make up water, rendering the necessity to purge air from the system not as critical as in pressurized installations. For instance, if there is a leak in a nonpressurized installation, the system owner (usually the homeowner) can easily add loop liquid to the system without the use of specialized tools or equipment. Nevertheless, although unpressurized systems are generally considered more forgiving than their pressurized counterparts, the same types of system failures can also occur. For instance, if too much air finds its way to the pump inlet, the pump can still become air locked and eventually overheat and fail. In addition, if there is inadequate fluid pressure at the suction side of the pump, cavitation can still result and the pump will eventually fail.
The present disclosure is directed toward avoiding pump failures in geothermal systems in general, and especially unpressurized geothermal systems specifically.